Switch controller, power supply device comprising the same, and driving method of the power supply device

ABSTRACT

The power supply device includes a power switch including one terminal to which an input voltage is transferred; an inductor including one terminal connected to another terminal of the power switch; a diode connected between a ground and a floating ground; a sensing resistor connected between the floating ground and the one terminal of the inductor. A switch controller compares a modulation sensing voltage depending on a sensing voltage generated from the sensing resistor with a high peak reference and a low peak reference when a LED string is connected between an inductor and the ground. The switch controller controls a switching operation of a power switch according to the comparison result. The high peak reference and the low peak reference are references for controlling an upper limit and a lower limit of an LED current flowing through the LED string, respectively.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2012-0048815 filed in the Korean IntellectualProperty Office on May 8, 2012, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

An exemplary embodiment of the present invention relates to a powersupply device implemented by a buck converter, a driving method of thepower supply device, and a switch controller applied to the power supplydevice.

(b) Description of the Related Art

FIG. 1 is a circuit diagram illustrating a conventional low-side buckconverter for driving an LED string.

As shown in FIG. 1, AC power source AC is rectified by passing through abridge diode 1. The bridge diode 1 full-wave rectifies the AC powersource AC. The rectified voltage, that is, an input voltage is suppliedto an inductor 2 through an LED string, and the inductor 2 supplies adriving current to the LED by an operation of a power switch S. Aswitching unit 3 including the power switch S controls a switchingoperation of the power switch S.

When the power switch S is turned-on, a LED current flowing through theLED string is increased. When the power switch S is turned-off, the LEDcurrent is reduced.

The switching unit 3 according to the conventional art controls aswitching operation according to a peak of a current flowing through thepower switch S. Since a current flows through a diode 4 in the LEDstring at an off period of the power switch S, a scheme of controllingonly a peak of a current flowing through the power switch S has alimitation on Constant Current (CC) control.

Further, sub-harmonic may be prevented in a current control mode bylimiting a switching duty to be 50% or less or performing slopecompensation. The slope compensation refers to varying a slope of acontrol signal for controlling a peak current flowing through the powerswitch. Then, there is a need for a complex circuit to limit a switchingduty or to perform the slope compensation.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a switchcontroller, a buck converter including the same, and a driving methodthereof having advantages of improving a CC control characteristic.

A power supply device according to an exemplary embodiment of thepresent invention includes: a power switch including one terminal towhich an input voltage is transferred; an inductor including oneterminal connected to another terminal of the power switch; a diodeconnected between a ground and a floating ground; a sensing resistorconnected between the floating ground and the one terminal of theinductor; and a switch controller controlling a switching operation ofthe power switch using a sensing voltage generated from the sensingresistor, and a high peak reference and a low peak reference forcontrolling an upper limit and a lower limit of an LED current flowingthrough an LED string when the LED string is connected between anotherterminal of the inductor and the ground.

The switch controller may inverting-amplify the sensing voltage, adds apredetermined offset voltage to the amplified voltage to generate amodulation sensing voltage, turns-on the power switch when themodulation sensing voltage reaches the low peak reference, and turns-offthe power switch when the modulation sensing voltage reaches the highpeak reference.

When the input voltage is generated by full-wave rectifying an AC input,the switch controller may detect a zero crossing time point of the inputsignal using the modulation sensing voltage, and generate the high peaksignal and the low peak signal having a phase and a waveform insynchronization with the input voltage using the detected zero crossingtime point.

The switch controller may detect a period when the modulation sensingvoltage is less than the low peak reference as a zero crossing detectionperiod, determine the zero crossing time during the detected zerocrossing detection period to set one period of the input voltage, andgenerate the high peak reference and the low peak reference depending ona full-wave rectification sine-wave during the one set period of theinput voltage.

The digital sine-wave generator may include a zero crossing detectorreceiving the modulation sensing voltage and the low peak reference,detecting a period when the modulation sensing voltage is less than thelow peak reference as the zero crossing detection period, and generatinga zero crossing detection signal according to the detected result; aclock generator receiving the zero crossing detection signal and apredetermined clock signal, estimating continuous zero crossing timepoints using the zero crossing detection signal, setting an intervalbetween the estimated continuous zero crossing time points as one periodof the input voltage, and generating a sine-wave clock signal with edgescorresponding to a reference number of times during the one set periodof the input voltage; a sine-wave generator generating a first digitalsignal and a second digital to be increased and reduced corresponding tothe reference number of times according to the sine-wave clock signalduring the one set period of the input voltage; and a digital-to-analogconverter converting the first digital signal and the second digitalsignal into analog voltage signals, respectively, to generate the highpeak reference and the low peak reference.

The clock generator may set an interval between one time during a periodwhen the zero crossing detection signal is at a low level and one timeduring a period when the zero crossing detection signal is at a next lowlevel as the one period of the input voltage.

The sine-wave generator may serially arrange a plurality of digitalvalues indicating a high peak reference varying for each period of thesine-wave clock signal for each period of the input voltage according tothe zero crossing detection signal to generate the first digital signal,and serially arrange a plurality of digital values indicating a low peakreference varying for each period of the sine-wave clock signal togenerate the second digital signal.

The switch controller may include a high peak comparator generating anOFF signal for controlling turning-off of the power switch according toa comparison result of the modulation sensing voltage with the high peakreference; a low peak comparator generating an ON signal for controllingturning-on of the power switch according to a comparison result of themodulation sensing voltage with the low peak reference; an SR flip-flopreceiving the OFF signal and the ON signal, generating a gate controlsignal of a level for turning-off the power switch according to the OFFsignal, and generating a gate control signal of a level for turning-onthe power switch according to the ON signal; and a gate drivergenerating a gate signal transferred to a gate electrode of the powerswitch according to the gate control signal.

The switch controller may include an inverting amplifier amplifying thesensing voltage −N times; and an offset adder adding the offset voltageto an output of the inverting amplifier to generate the modulationsensing voltage.

The sensing voltage may be a voltage generated from the sensing resistorbased on the floating ground.

A method of driving a power supply device according to an exemplaryembodiment of the present invention relates to a power supply deviceincluding a power switch having one terminal receiving an input voltage,a diode connected between another terminal of the power switch and aground, and a sensing resistor between the another terminal of the powerswitch and an inductor. The method of driving a power supply deviceincludes comparing a modulation sensing voltage according to a sensingvoltage generated from the sensing resistor with a high peak referencevoltage when an LED string is connected between the inductor and theground the inductor; comparing the modulation sensing voltage with a lowpeak reference; and turning-off the power switch according to thecomparison result of the modulation sensing voltage with the high peakreference, and turning-on the power switch according to the comparisonresult of the modulation sensing voltage with the low peak reference,wherein the high peak reference and the low peak reference arereferences for controlling an upper limit and a lower limit of an LEDcurrent flowing through the LED string, respectively.

The method of driving a power supply device may further includeinverting-amplifying the sensing voltage, and adding a predeterminedoffset voltage to the amplified voltage to generate the modulationsensing voltage.

The method of driving a power supply device may further includedetecting a zero crossing detection period according to the comparisonresult of the modulation sensing voltage with the low peak reference;determining a zero crossing time point during the detected zero crossingdetection period, and setting continuous zero crossing time points asone period of the input voltage and generating the high peak referenceand the low peak reference depending on a full-wave sine wave during theone set period of the input voltage.

The detecting of the zero crossing detection period may includedetecting a period when the modulation sensing voltage is less than thelow peak reference as the zero crossing detection period.

The generating of the high peak reference and the low peak reference mayinclude: generating a sine-wave clock signal with edges corresponding toa reference number of times during the one set period of the inputvoltage; generating a first digital signal and a second digital to beincreased and reduced corresponding to the reference number of timesaccording to the sine-wave clock signal during the one set period of theinput voltage; and converting the first digital signal and the seconddigital signal into analog voltage signals, respectively, to generatethe high peak reference and the low peak reference.

A switch controller according to an exemplary embodiment of the presentinvention is applicable to a power supply device including a diodesupplying a current to an LED string according to a switching operationof a power switch including one terminal to which an input voltage issupplied, the diode being connected between another terminal of thepower switch and a ground. The switch controller includes an invertingamplifier inverting-amplifying a sensing voltage generated from asensing resistor connected to the another terminal of the power switch;and an offset adder adding the offset voltage to an output of theinverting amplifier to generate the modulation sensing voltage, whereina switching operation of the power switch is controlled using themodulation sensing voltage, and a high peak reference and a low peakreference for controlling an upper limit and a lower limit of an LEDcurrent flowing through the LED string.

An exemplary embodiment of the present invention provides a switchcontroller capable of improving a CC control characteristic, a buckconverter including the same, and a driving method thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a low-side buck converter fordriving an LED string.

FIG. 2 is a circuit diagram illustrating a switch controller and a powersupply device including the same according to an exemplary embodiment ofthe present invention.

FIG. 3 is a circuit diagram illustrating a digital sine-wave generator(DSG) according to an exemplary embodiment of the present invention.

FIG. 4 is waveform diagram illustrating a modulation sensing voltage, azero crossing detection signal, a sine-wave clock signal, a high peakreference, and a low peak reference generated by the exemplaryembodiment of the present invention.

FIG. 5 is a waveform diagram illustrating a high peak reference, a lowpeak signal, a modulation sensing voltage, a gate signal, a sensingvoltage, an LED current, and a switching frequency according to theexemplary embodiment of the present invention.

FIG. 6 is a circuit diagram illustrating a switch controller accordingto another exemplary embodiment of the present invention.

FIG. 7 is a waveform diagram illustrating a high peak reference, a lowpeak signal, a modulation sensing voltage, a gate signal, a sensingvoltage, an LED current, and a switching frequency according to anotherexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

Throughout this specification and the claims that follow, when it isdescribed that an element is “coupled” to another element, the elementmay be “directly coupled” to the other element or “electrically coupled”to the other element through a third element. In addition, unlessexplicitly described to the contrary, the word “comprise” and variationssuch as “comprises” or “comprising”, will be understood to imply theinclusion of stated elements but not the exclusion of any otherelements.

Hereinafter, an exemplary embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 2 is a circuit diagram illustrating a switch controller and a powersupply device including the same according to an exemplary embodiment ofthe present invention.

The power supply device according to the exemplary embodiment of thepresent invention is implemented by a high-side buck converter. In thehigh-side buck converter, a drain electrode of a power switch M isdirectly connected to an input voltage VIN.

The input voltage VIN according to the exemplary embodiment of thepresent invention is a voltage generated by full-wave rectifying an ACinput AC. Different from the high-side buck converter according to theexemplary embodiment of the present invention, the power switch S of thelow-side buck according to the conventional art is connected between aground and a LED string.

As shown in FIG. 2, the power switch M performs a switching operationaccording to a gate signal VG received from the switch controller 30.The power switch M is implemented by an n-channel metal oxidesemiconductor filed effect transistor (NMOSFET). The present inventionis not limited thereto, but another type of transistor is applicable tothe present invention.

A power supply device 40 includes a power switch M, a bridge diode 10, adiode D, an inductor L, and a switch controller 30. The switchcontroller 30 and the power switch M are integrated with each other in apackage as one chip.

The bridge diode 10 includes four diodes 11 to 14, and full-waverectifiers an input AC power source AC to generate an input voltage VIN.The input voltage VIN has a full-wave rectification waveform.

The power switch M includes a drain electrode connected to an outputterminal of the bridge diode 11, a gate electrode to which a gate signalVG from the switch controller 30 is input, and a source electrodeconnected to a floating ground FGND.

The LED string 20 includes a plurality of LED devices which areconnected to each other in series.

A sensing resistor RS includes one terminal connected to the floatingground FGND and the other terminal connected to one terminal of theinductor L. A sensing voltage VCS generated by passing a current(hereinafter referred to as ‘LED current ILED’) flowing through the LEDstring 20 through the sensing resistor RS is used to control zerocrossing detection of the input voltage VIN and a switching operation.Since the sensing voltage VCS has a potential lower than that of thefloating ground FGND, the sensing voltage VCS is always a negativevoltage.

The other terminal of the inductor L is connected to one terminal of theLED string 20, and the other terminal of the LED string 20 is connectedto a ground. Accordingly, the current (IL) flowing through the inductorL is supplied to the LED string 20, and the LED 20 emits light accordingto the inductor current IL.

The diode D is connected between the floating ground FGND and theground. The current flowing through the LED string 20 passes through thediode D during an off period of the power switch M.

If the power switch M is turned-on, energy starts to be accumulated inthe inductor L according to an input voltage VIN. During an on period ofthe power switch M, the energy is accumulated in the inductor L, and theinductor current IL is increased. In this case, the inductor current ILflows to the ground through the LED string 20.

If the power switch M is turned-off, the energy accumulated in theinductor L starts to be reduced. During an off period of the powerswitch M, the energy in the inductor L is reduced so that the inductorcurrent IL is reduced. In this case, the inductor current IL isfree-wheeled through the LED string 20 and the diode D.

The switch controller 30 detects a zero crossing time point of the inputvoltage VIN using the sensing voltage VCS, and generates a high peaksignal VPH and a low peak signal VPL having a phase and a waveform insynchronization with the input voltage VIN using the detected zerocrossing time point. The switch controller 30 controls a switchingoperation of the power switch M using the sensing voltage VCS, the highpeak signal VPH, and the low peak signal VPL.

The switch controller 30 includes a digital sine-wave generator(hereinafter referred to as ‘DSC’) 100, a high peak comparator 200, alow peak comparator 250, an SR flip-flop 300, gate driver 350, aninverting amplifier 400, and an offset adder 450.

In a buck converter, an output voltage VOUT is controlled to be equal toor less than the input voltage VIN. In this case, the output voltageVOUT is a value n*VF obtained by multiplying the number n of LEDSconstituting the LED string by a forwarding voltage VF, and theforwarding voltage VF refers to a voltage difference across the LED whena current flows through the LED.

If the input voltage VIN is less than a desired output voltage VOUT, theoutput voltage is not controlled to a described level and an outputcurrent does not flow to an output terminal. Accordingly, the currentdoes not flow through the sensing resistor RS so that the sensingvoltage VCS has the same potential as that of the floating ground FGND.

In then exemplary embodiment of the present invention, the outputcurrent, that is, the LED current ILED is the same as the currentflowing through the sensing resistor RS. Accordingly, the output current(or LED current) is detected as the sensing voltage VCS detected by thesensing resistor RS.

For example, when the input voltage VIN is approximately a zero voltage,the input voltage VIN is less than an output voltage VOUT of desirelevel so that the LED current ILED is not generated. Accordingly, thesensing voltage VCS has the same potential as that of the floatingground FGND.

The switch controller 30 inverting-amplifies the sensing voltage VCS andadds the predetermined offset voltage VOFS to the amplified voltage togenerate a modulation sensing voltage VCSN. The modulation sensingvoltage VCSN and the LED current ILED have a similar waveform.Accordingly, when the input voltage VIN is approximately the zerovoltage, the modulation sensing voltage VCSN has the lowermostpotential. The exemplary embodiment of the present invention may detecta zero voltage crossing time point using the modulation sensing voltageVCSN.

In detail, the inverting amplifier 400 amplifies the sensing voltage VCS−N times, and the offset adder 450 adds an offset voltage VOFS to theoutput of the inverting amplifier 400 to generate the modulation sensingvoltage VCSN.

The switch controller 30 detects a period (hereinafter referred to as‘zero crossing detection period’) when the modulation sensing voltageVCSN is less than the low peak reference VPL, and may determine a middletime of the zero crossing detection period as the zero crossing timepoint. A method of determining the zero crossing time point during thezero crossing detection period by the switch controller 30 may bechanged. For example, the switch controller 30 may determine an optionaltime point, a start time point, or an end time point of the zerocrossing detection period as the zero crossing time point.

The DSG 100 detects the zero crossing detection period using themodulation sensing voltage VCSN, determines the zero crossing time pointduring the detected zero crossing detection period to set one a periodof the input voltage VIN, and generates a high peak reference VPH andthe low peak reference VPL depending on a full rectification sine-waveduring one period of a set input voltage VIN.

The peak reference VPH is a reference to control a high peak of acurrent (hereinafter referred to as ‘LED current’) flowing through theLED string 20, and the low peak reference VPL is a reference to controla low peak of the LED current ILED.

The high peak refers to an upper limit of an LED current ILED increasedduring an on period of the power switch M, and The low peak refers to alower limit of an LED current ILED reduced during an off period of thepower switch M.

Referring to FIGS. 3 and 4, the DSG 100 according to the exemplaryembodiment of the present invention will be described.

FIG. 3 is a circuit diagram illustrating a DSG according to an exemplaryembodiment of the present invention.

FIG. 4 is waveform diagram illustrating a modulation sensing voltage, azero crossing detection signal, a sine-wave clock signal, a high peakreference, and a low peak reference generated by the exemplaryembodiment of the present invention.

As shown in FIG. 3, the DSG 100 includes a zero crossing detector 110, aclock generator 120, a sine-wave generator 130, and a digital-analogconverter (DAC) 140.

The zero crossing detector 110 receives a modulation sensing voltageVCSN and a low peak reference VPL, detects a zero crossing detectionperiod when the modulation sensing voltage VCSN is less than the lowpeak reference VPL, and generates a zero crossing detection signalZCD_OUT according to the detected result.

A ZCD_P1 being a part of the first zero crossing detection period, thesecond zero crossing detection period ZCD_P2, and the third zerocrossing detection period ZCD_P3 are illustrated in FIG. 4. A zerocrossing detection signal ZCD_OUT according to the exemplary embodimentof the present invention is at a low level during the zero crossingdetection period. The zero crossing detection signal ZCD_OUT is at ahigh level during remaining periods.

The clock generator 120 receives the zero crossing detection signalZCD_OUT and a predetermined clock signal CLK, sets one period of aninput voltage VIN using the zero crossing detection signal ZCD_OUT, andgenerates a sine-wave clock signal CLKG having a predetermined frequencyduring the one set period.

The clock generator 120 estimates continuous zero crossing time pointsusing the zero crossing detection signal ZCD_OUT, and sets an intervalbetween the estimated zero crossing time points as one period of theinput voltage VIN. For example, the clock generator 120 estimates onetime point during a period when the zero crossing detection signalZCD_OUT is at a low level as the zero crossing time point, estimates onetime point during a period when the zero crossing detection signalZCD_OUT is at a high level as a next zero crossing time point, and setsan interval between the zero crossing time point and the next zerocrossing time point as one of the input periods (VIN).

The clock generator 120 generates a sine-wave clock signal CLKG having apredetermined number of edges during one period of the predeterminedinput voltage VIN. In detail, the clock generator 120 divides apredetermined clock signal CLK to generate the sine-wave clock signalCLKG having a predetermined number of edges during one period of anestimated input voltage VIN. In this case, the predetermined number maybe suitably set so that the high peak reference VPH and the low peakreference VPL have a sine-wave similar to that of the input voltage VIN.Hereinafter, the number of edges of the sine-wave clock signal CLKGduring one period of the input voltage VIN refers to a reference numberof times.

For example, a reduced modulation sensing voltage (VCSN) is set to reacha peak reference VPL at a time point T0, and a voltage VCSN increasedaccording to increase of the input voltage VIN after the zero voltagecrossing is set to reach the low peak reference VPL at a time point T1.

Further, the modulation sensing voltage VCSN is set to reach the lowpeak reference VPL at time points T2 and T3, and is set to reach the lowpeak reference VPL at time points T4 and T5. The clock generator 120according to the exemplary embodiment of the present invention sets toestimate a middle time point of the zero crossing detection period asthe zero crossing time point.

The clock generator 120 sets periods TO1 to T23 as one period of theinput voltage VIN, and generates a sine-wave clock signal CLKG of afrequency having a rising edge of a reference number of times (e.g., 22times) during the one set period after a time point T23. That is, afrequency of a clock signal CLKG during periods T23 to T45 is determinedaccording to a period (T01 to T23) just before the input signal VIN.

During periods TO1 to T23, a frequency of a sine-wave clock signal CLKGgenerated form the clock generator 120 is determined according to aperiod just before the input signal VIN based on the time point T01.

The sine-wave generator 130 generates a first digital signal DS1 and thesecond digital signal DS2 increased and reduced by the reference numberof times for each period of the set input voltage VIN according to thesine-wave clock signal CLKG. The sine-wave generator 130 seriallyarranges a plurality of digital values indicating a high peak referenceVPH varying for each period of the sine-wave clock signal CLKG togenerate the first digital signal DS1, and serially arranges a pluralityof digital values indicating a low peak reference VPL varying for eachperiod of the sine-wave clock signal CLKG to generate the second digitalsignal DS2.

The sine-wave generator 130 may receive a zero crossing detection signalZCD_OUT in order to generate the first digital signal DS1 and the seconddigital signal DS2 for each period of the input voltage VIN.

For example, each of the first digital signal DS1 and the second digitalsignal DS2 may be a digital signal in n bit units. That is, each of thehigh peak reference VPH and the low peak reference VPL varying insynchronization with the sine-wave clock signal CLKG is denoted as an nbit digital value. An increase amount or a reduction amount of each ofthe first digital signal DS1 and the second digital signal DS2 is set tomaintain an interval as illustrated in FIG. 4 as each of the high peakreference VPH and the low peak reference VPL implements a sine-wave.

The DAC 140 converts the first input digital signal DS1 and the secondinput digital signal DS2 into analog voltage signals in real time togenerate and output the high peak reference VPH and the low peakreference VPL.

Then, as shown in FIG. 4, the high peak reference VPH and the low peakreference VPL are generated depending on a sine-wave, which areincreased or reduced in synchronization with a rising edge of thesine-wave clock signal CLKG.

As shown in FIG. 4, during a period from a time point T11 when a firstedge E1 of a sine-wave clock signal GCLK generated after the zerocrossing time point T10 is generated to a time point T12 when aneleventh edge is generated, the first digital signal DS1 and the seconddigital signal DS2 are increased, and the peak reference VPH and the lowpeak reference VPL are increased in synchronization with a rising edgeof the sine-wave clock signal CLKG.

In the exemplary embodiment of the present invention, at the time pointT11, the first digital signal DS1 and the second digital signal DS2 arenot increased, and the high peak reference VPH and the low peakreference VPL are not increased. A period maintaining a constant valueis set in order to detect the zero crossing detection period of theinput voltage VIN using the high peak reference VPH and the low peakreference VPL.

During a period from a generation time point T13 of an twelfth edge E12to a generation time point T15 of a twenty-second edge, the firstdigital signal DS1 and the second digital signal DS2 are reduced, andthe peak reference VPH and the low peak reference VPL are reduced insynchronization with a rising edge of the sine-wave clock signal CLKG.

In the exemplary embodiment of the present invention, at the time pointT13, the first digital signal DS1 and the second digital signal DS2 arenot reduced, and the high peak reference VPH and the low peak referenceVPL are not reduced. This is change of a design to implement a waveformsimilar to a sine wave, but the present invention is not limitedthereto.

The DSG 100 according to the exemplary embodiment of the presentinvention sets a middle time point of the zero crossing detection periodor a rising edge or a falling edge of the zero crossing detection signalZCD_OUT, or an optional time point of the zero crossing detection periodas the zero crossing time point. The zero crossing detection period isvery short, and an optional time point, a rising edge time point, and afalling edge time point there may be very close to each other in thetime.

The high peak comparator 200 generates an OFF signal for controllingturning-off of the power switch M according to a comparison result ofthe modulation sensing voltage VCSN with and the high peak referenceVPH.

The modulation sensing voltage VCSN is input to a non-inverting terminal(+) of the high peak comparator 200, and the high peak reference VPH isinput to an inverting terminal (−) of the high peak comparator 200. If asignal input to the non-inverting terminal (+) is equal or greater thana signal input to the inverting terminal (−), the high peak comparator200 outputs an OFF signal of high level. Otherwise, the high peakcomparator 200 outputs an OFF signal of low level.

The low peak comparator 250 generates an ON signal for controllingturning-on of the power switch M according to a comparison result of themodulation sensing voltage VCSN with and the low peak reference VPL.

The modulation sensing voltage VCSN is input to an inverting terminal(−) of the low peak comparator 250, and the low peak reference VPL isinput to a non-inverting terminal (+) of the low peak comparator 250. Ifa signal input to the non-inverting terminal (+) is equal or greaterthan a signal input to the inverting terminal (−), the low peakcomparator 250 outputs an ON signal of high level. Otherwise, the lowpeak comparator 250 outputs an ON signal of low level.

The SR flip-flop 300 generates a gate control signal VGC of a level forcontrolling turning-on of the power switch M according to an ON signal,and generates a gate control signal VGC of a level for controllingturning-off of the power switch M according to an OFF signal.

If a signal input to a set terminal S is at a high level, the SRflip-flop 300 generates a gate control signal VGC of a low level andoutputs the generated gate control signal VGC through an invertingoutput terminal Qb. If a signal input to a reset terminal R is at a highlevel, the SR flip-flop 300 generates a gate control signal VGC of ahigh level and outputs the generated gate control signal VGC through theinverting output terminal Qb. The ON signal is input to a reset terminalR of the SR flip-flop 300, and the OFF signal is input to the setterminal S of the SR flip-flop 300.

Accordingly, when the modulation sensing voltage VCSN reaches the highpeak reference VPH so the OFF signal of a high level is generated, theSR flip-flop 300 generates a gate control signal VGC of a low level.When the modulation sensing voltage VCSN reaches the low peak referenceVPL so the ON signal of a high level is generated, the SR flip-flop 300generates a gate control signal VGC of a high level.

The gate driver 350 generates a gate signal VG for controlling aswitching operation of the power switch M according to a gate controlsignal VGC. When the gate signal VG is at a high level, the power switchM is turned-on. When the gate signal VG is at a low level, the powerswitch M is turned-off. The gate driver 350 generates a gate signal VGof a high level according to a gate control signal VGC of a high level,and generates a gate signal VG of a low level according to a gatecontrol signal VGC of a low level.

FIG. 5 is a waveform diagram illustrating a high peak reference, a lowpeak signal, a modulation sensing voltage, a gate signal, a sensingvoltage, an LED current, and a switching frequency according to theexemplary embodiment of the present invention.

An LED current ILED is increased during a high level period (On periodof power switch M) of the gate signal VG, and is reduced during a lowlevel period (OFF period of power switch M) of the gate signal VG, andthe sensing voltage VCS varies with the LED current ILED. However, thesensing voltage VCS is a negative voltage, which varies with variationand a reverse phase of the LED current ILED.

That is, as shown in FIG. 5, while the LED current ILED is increased, anegative sensing voltage VCS is reduced. While the LED current ILED isreduced, the negative sensing voltage VCS is increased.

The sensing voltage VCS is amplified −N times and an offset voltage VOFSis added to the amplified voltage to generate a modulation sensingvoltage VCSN as shown in FIG. 5. That is, the modulation sensing voltageVCSN is increased during an ON period of the power switch M. Themodulation sensing voltage VCSN is reduced during an OFF period of thepower switch M.

At a time point (e.g., T21) when the modulation sensing voltage VCSNreaches the high peak reference VPH, the gate signal VG becomes a lowlevel so that the power switch M is turned-off. Then, from the timepoint T21, the LED current ILED is reduced, the negative sensing voltageVCS is increased, and the modulation sensing voltage VCSN is reduced.

When the reduced modulation detection modulation sensing voltage VCSNreaches the low peak reference VPL at a time point (e.g., T22), the gatesignal VG becomes a high level so that the power switch M is turned-on.Then, from the time point T22, the LED current ILED is increased, thenegative sensing voltage VCS is reduced, but the modulation sensingvoltage VCSN is increased.

When the increased modulation detection modulation sensing voltage VCSNreaches the high peak reference VPH at a time point (e.g., T23), thegate signal VG becomes a low level so that the power switch M isturned-off. Then, from the time point T23, the LED current ILED isreduced, the negative sensing voltage VCS is increased, but themodulation sensing voltage VCSN is reduced.

In this manner, a switching operation is controlled according to themodulation sensing voltage VCSN, the peak reference VPH, and the lowpeak reference VPL.

However, among periods T20 to T21 and T23, during a period inkling azero crossing detection period, the switch M is turned-on according to agate signal VG of a high level, the input voltage VIN is approximately azero voltage. Accordingly, a period when the modulation sensing voltageVCSN does not vary with a switching operation is generated.

For example, the modulation sensing voltage VCSN reaches a low peakreference VPL at a time point T24 so that the power switch M isturned-on according to the gate signal VG of a high level, but themodulation sensing voltage VCSN is reduced without increase.

Further, as the input voltage VIN is increased, a switching frequencyFosc is reduced. As the input voltage VIN is reduced, the switchingfrequency Fosc is increased.

In this manner, the power supply device according to the exemplaryembodiment of the present invention may estimate a zero crossing timepoint of an input voltage using a sensing voltage depending on an LEDcurrent, and set a high peak reference and a low peak referencedepending on a phase and a waveform of an input voltage, so that rippleof the LED current can be controlled. Accordingly, a CC characteristicis improved.

In addition, during one period of the input voltage, an interval betweenthe high peak reference and the low peak reference may be changedaccording to magnitude of the input voltage. When the input voltage ishigh during one period of the input voltage, the interval between thehigh peak reference and the low peak reference is wide. When the inputvoltage is low during one period of the input voltage, the intervalbetween the high peak reference and the low peak reference is narrow.Accordingly, the switching frequency varies so that an EMIcharacteristic is improved.

Different from the buck converter according to the conventional artwhere a duty is limited to 50% or slope compensation must be performed,the present invention may use a duty of 50% or greater and does notrequire separate slope compensation.

Further, since a voltage across an LED string according to theconventional art is a high voltage, it difficult to detect the voltageacross the LED string. However, in the exemplary embodiment of thepresent invention, since the LED string is connected to the floatingground, it is easy to detect the voltage across the LED string.

The foregoing embodiment has illustrates a case where an input of thepower supply device is AC.

Hereinafter, a case where an input of the power supply device is DC willbe described. When the input of the power supply device is DC, an inputvoltage VIN has a constant value. Accordingly, when the input of thepower supply device is DC, the high peak reference and the low peakreference does not depend on a sine-wave but have a constant value.

In a case of a DC input, the switch controller 30 sets the high peakreference and the low peak reference to predetermined values,respectively. When the modulation sensing voltage VCSN reaches the highpeak reference, the switch controller 30 turns-off the power switch M.When the modulation sensing voltage VCSN reaches the low peak reference,the switch controller 30 turns-on the power switch M.

FIG. 6 is a circuit diagram illustrating a switch controller accordingto another exemplary embodiment of the present invention.

Upon comparing a switching controller 30′ shown in FIG. 6 with theswitching controller 30 according to the exemplary embodiment, thedifference is that a high peak reference VPH′ and a low peak referenceVPL′ are fixed to a constant voltage according to a DC input. The samecomponents will be assigned with the same reference numerals.

Although a digital sine-wave generator is not shown in FIG. 6, thedigital sine-wave generator may be included in the switch controller30′. In the case of the DC input, a zero crossing detection signalZCD_OUT is always held at a high level (or low level), and the digitalsine-wave generator may output a constant high peak reference VPH′ and aconstant low peak reference VPL′ rather than a digital sine wave.

FIG. 7 is a waveform diagram illustrating a high peak reference, a lowpeak signal, a modulation sensing voltage, a gate signal, a sensingvoltage, an LED current, and a switching frequency according to anotherexemplary embodiment of the present invention.

As shown in FIG. 7, if a modulation sensing voltage VCSN reaches thehigh peak reference VPH′ at a time point T31, a gate signal VG becomes alow level according to an OFF signal of a high level. Accordingly, theswitch M is turned-off.

Then, from the time point T31, an LED current ILED is reduced, anegative sensing voltage VCS is increased, but the modulation sensingvoltage VCSN is reduced.

When the reduced modulation detection modulation sensing voltage VCSNreaches the low peak reference VPL at a time point T32, the gate signalVG becomes a high level according to an ON signal of a high level.Accordingly, the power switch M is turned-on.

Then, from the time point T32, the LED current ILED is increased, thenegative sensing voltage VCS is reduced, but the modulation sensingvoltage VCSN is increased.

When the increased modulation detection modulation sensing voltage VCSNreaches the high peak reference VPH′ at a time point T33, the gatesignal VG becomes a low level according to an OFF signal of a highlevel. Accordingly, the power switch M is turned-off.

Then, from the time point T33, the LED current ILED is reduced, thenegative sensing voltage VCS is increased, but the modulation sensingvoltage VCSN is reduced.

In this manner, since the LED current ILED is controlled according tothe peak reference VPH and the low peak reference VPL which are fixed toa constant value, a switching frequency Fosc has a constant value.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

DESCRIPTION OF SYMBOLS

-   Power supply device 40, power switch M, bridge diode 10 diode (D) 11    to 14, inductor L, switch controller 30, LED string 20 digital    sine-wave generator 100, high peak comparator 200, low peak    comparator 250-   SR flip-flop 300, gate driver 350, inverting amplifier 400 offset    adder 450, zero crossing detector 110, clock generator 120-   Sine-wave generating unit 130, digital-analog converter 140

What is claimed is:
 1. A power supply device comprising: a power switchincluding a first terminal and a second terminal, the first terminalbeing configured to receive an input voltage, the input voltage having arectified Alternating Current (AC) voltage component; an inductorincluding a first terminal and a second terminal, the first terminalbeing coupled to the second terminal of the power switch; a diodecoupled between a ground and a floating ground; a sensing resistorcoupled between the floating ground and the first terminal of theinductor; a digital sine-wave generator configured to detect a zerocrossing detection period, determine a zero crossing time during thedetected zero crossing detection period to set one period of the inputvoltage, and generate a first reference and a second reference based ona full-wave rectification sine-wave during the one set period of theinput voltage; and a switch controller configured to control a switchingoperation of the power switch based on a sensing voltage generated bythe sensing resistor, the first reference, and the second reference,wherein the first reference and the second reference are to control anupper limit and a lower limit, respectively, of an LED current flowingthrough an LED string when the LED string is coupled between the secondterminal of the inductor and the ground, wherein the first reference hasa first waveform including an AC component, the AC component of thefirst waveform having a phase synchronized with a phase of the rectifiedAC voltage component of the input voltage, and wherein the secondreference has a second waveform including an AC component, the ACcomponent of the second waveform having a phase synchronized with thephase of the rectified AC voltage component of the input voltage.
 2. Thepower supply device of claim 1, wherein the switch controller isconfigured to invert-amplify the sensing voltage and add a predeterminedoffset voltage to the amplified sensing voltage to generate a modulationsensing voltage, wherein the switch controller is configured to turn onthe power switch when the modulation sensing voltage reaches the secondreference and turn off the power switch when the modulation sensingvoltage reaches the first reference.
 3. The power supply device of claim2, wherein the switch controller is configured to detect the zerocrossing time point of the input voltage based, at least in part, on themodulation sensing voltage.
 4. The power supply device of claim 2,wherein the digital sine-wave generator is configured to detect a periodwhen the modulation sensing voltage is less than the second reference asthe zero crossing detection period.
 5. The power supply device of claim4, wherein the digital sine-wave generator comprises: a zero crossingdetector configured to receive the modulation sensing voltage and thesecond reference, detect a period when the modulation sensing voltage isless than the second reference as the zero crossing detection period andgenerate a zero crossing detection signal according to the detectedresult; a clock generator configured to receive the zero crossingdetection signal and a predetermined clock signal, estimate continuouszero crossing time points based, at least in part, on the zero crossingdetection signal, set an interval between the estimated continuous zerocrossing time points as one period of the input voltage, and generate asine-wave clock signal with edges corresponding to a reference number oftimes during the one set period of the input voltage; a sine-wavegenerator configured to generate a first digital signal and a seconddigital to be increased and reduced corresponding to the referencenumber of times according to the sine-wave clock signal during the oneset period of the input voltage; and a digital-to-analog converterconfigured to convert the first digital signal and the second digitalsignal into analog voltage signals, respectively, to generate the firstreference and the second reference.
 6. The power supply device of claim5, wherein the clock generator is configured to set an interval betweenone time during a period when the zero crossing detection signal is at alow level and one time during a period when the zero crossing detectionsignal is at a next low level as the one period of the input voltage. 7.The power supply device of claim 5, wherein the sine-wave generator isconfigured to serially arrange a plurality of digital values indicatingthe first reference varying for each period of the sine-wave clocksignal for each period of the input voltage based on the zero crossingdetection signal to generate the first digital signal, the sine-wavegenerator being further configured to serially arrange a plurality ofdigital values indicating the second reference varying for each periodof the sine-wave clock signal to generate the second digital signal. 8.The power supply device of claim 4, wherein the switch controllercomprises: a high peak comparator configured to generate an OFF signalfor turning off the power switch based on a result of a comparisonbetween the modulation sensing voltage and the first reference; a lowpeak comparator configured to generate an ON signal for turning on thepower switch based on a result of a comparison between the modulationsensing voltage and the second reference; an SR flip-flop configured toreceive the OFF signal and the ON signal and generate a first gatecontrol signal configured to turn off the power switch based on the OFFsignal and generate a second gate control signal configured to turn onthe power switch based on the ON signal; and a gate driver configured togenerate a gate signal to transfer to a gate electrode of the powerswitch based on at least one of the first and second gate controlsignal.
 9. The power supply device of claim 4, wherein the switchcontroller comprises: an inverting amplifier configured to amplify thesensing voltage N times; and an offset adder configured to add theoffset voltage to an output of the inverting amplifier to generate themodulation sensing voltage.
 10. The power supply device of claim 1,wherein the sensing voltage is a voltage generated by the sensingresistor based on the floating ground.
 11. A method of driving a powersupply device, the method comprising: providing a power supply devicecomprising: a power switch having a first terminal and a secondterminal, the first terminal receiving an input voltage, the inputvoltage having a rectified Alternating Current (AC) voltage component; adiode coupled between the second terminal of the power switch and aground; and a sensing resistor coupled between the second terminal ofthe power switch and an inductor; generating a first reference having afirst waveform including an AC component, the AC component of the firstwaveform having a phase synchronized with a phase of the rectified ACvoltage component of the input voltage; generating a second referencehaving a second waveform including an AC component, the AC component ofthe second waveform having a phase synchronized with the phase of therectified AC voltage component of the input voltage; comparing amodulation sensing voltage that is based on a sensing voltage generatedby the sensing resistor with the first reference when an LED string isconnected between the inductor and the ground; comparing the modulationsensing voltage with the second reference; detecting a zero crossingdetection period based on the comparison of the modulation sensingvoltage with the second reference; determining a zero crossing timepoint during the detected zero crossing detection period; determining aone set period of the input voltage using the zero crossing time point,generating a clock signal with edges corresponding to a reference numberof times during the one set period of the input voltage; generating afirst digital signal configured to be increased and reducedcorresponding to the reference number of times based on the clock signalduring the one set period of the input voltage; and generating a seconddigital signal configured to be increased and reduced corresponding tothe reference number of times based on the clock signal during the oneset period of the input voltage; and turning off the power switch basedon the comparison of the modulation sensing voltage with the firstreference and turning on the power switch based on the comparison of themodulation sensing voltage with the second reference, wherein the firstreference and the second reference are references for controlling anupper limit and a lower limit of an LED current flowing through the LEDstring, respectively, wherein generating the first reference includesconverting the first digital signal into a first analog voltage signaland generating the first reference using the first analog voltagesignal, and wherein generating the second reference includes convertingthe second digital signal into a second analog voltage signal andgenerating the second reference using the second analog voltage signal.12. The method of claim 11, further comprising invert-amplifying thesensing voltage and adding a predetermined offset voltage to theamplified voltage to generate the modulation sensing voltage.
 13. Themethod of claim 12, further comprising: wherein generating the firstreference and the second reference is each based on a rectified sinewave during the one set period of the input voltage.
 14. The method ofclaim 13, wherein the detecting of the zero crossing detection periodcomprises: detecting a period when the modulation sensing voltage isless than the second reference as the zero crossing detection period.15. A switch controller of a power supply device including a diodesupplying a current to an LED string based on a switching operation of apower switch including a first terminal to which an input voltage issupplied, the diode being coupled between a second terminal of the powerswitch and a ground, the switch controller comprising: an invertingamplifier configured to invert-amplify a sensing voltage generated froma sensing resistor coupled to the second terminal of the power switch;an offset adder configured to add the offset voltage to an output of theinverting amplifier to generate a modulation sensing voltage; and adigital sine-wave generator configured to detect a period when themodulation sensing voltage is less than a second reference as a zerocrossing detection period and further determine a zero crossing timepoint during the detected zero crossing detection period to set oneperiod of the input voltage, the digital sine-wave generator beingfurther configured to generate a first reference and a second referencebased on a full-wave rectification sine-wave during the one set periodof the input voltage, wherein a switching operation of the power switchis controlled based on the modulation sensing voltage, the firstreference and the second reference, wherein the first reference and thesecond reference are to control an upper limit and a lower limit of anLED current flowing through the LED string, respectively, wherein thefirst reference has a first waveform including an AC component, the ACcomponent of the first waveform having a phase synchronized with a phaseof a rectified AC voltage component of the input voltage, and whereinthe second reference has a second waveform including an AC component,the AC component of the second waveform having a phase synchronized withthe phase of the rectified AC voltage component of the input voltage.16. The switch controller of a power supply device of claim 15, wherein:the switch controller is configured to turn on the power switch when themodulation sensing voltage reaches the second reference and configuredto turn off the power switch when the modulation sensing voltage reachesthe first reference.
 17. The switch controller of a power supply deviceof claim 16, wherein, when the input voltage is generated by full-waverectifying an AC input, the switch controller is configure to detect azero crossing time point of the input voltage based, at least in part,on the modulation sensing voltage and generate the first reference andthe second reference based, at least in part, on the detected zerocrossing time point.
 18. The switch controller of a power supply deviceof claim 15, wherein the digital since wave generator comprises: a zerocrossing detector configured to receive the modulation sensing voltageand the second reference, detect a period when the modulation sensingvoltage is less than the second reference as the zero crossing detectionperiod and generate a zero crossing detection signal according to thedetected result; a clock generator configured to receive the zerocrossing detection signal and a predetermined clock signal, estimatecontinuous zero crossing time points based, at least in part, on thezero crossing detection signal, set an interval between the estimatedcontinuous zero crossing time points as one period of the input voltageand generate a sine-wave clock signal with edges corresponding to areference number of times during the one set period of the inputvoltage; a sine-wave generator configured to generate a first digitalsignal and a second digital to be increased and reduced corresponding tothe reference number of times according to the sine-wave clock signalduring the one set period of the input voltage; and a digital-to-analogconverter configured to convert the first digital signal and the seconddigital signal into analog voltage signals, respectively, to generatethe first reference and the second reference.